In recent years, a configuration in which each of pixels has a charge holding portion separated from a photoelectric conversion portion and a floating diffusion (hereinafter FD) has been considered for higher performance of a solid-state image pickup device. The charge holding portion is provided in each pixel in order to realize a global electronic shutter, as described in U.S. Pat. No. 7,414,233 and Japanese Patent Laid-Open No. 2008-004692, or in order to expand a dynamic range, as described in Japanese Patent Laid-Open No. 2006-197383. Furthermore, the charge holding portion is provided in each pixel also in a configuration in which each pixel has an analog-to-digital (AD) converter, as described in Japanese Patent Laid-Open No. 2009-038167.
U.S. Pat. No. 7,414,233 discloses a configuration in which a P-type semiconductor region is disposed on a surface portion so as to separate an electrode TX1 on a charge holding portion formed of an N-type semiconductor region and an electrode TX2 for transferring charges in the charge holding portion from each other. Such a configuration prevents a dark current flowing to an accumulation region.
However, according to the configuration of U.S. Pat. No. 7,414,233, a P-type semiconductor region is disposed so as to separate two transfer gate structures from each other, and an N-type semiconductor region constituting a charge holding portion is disposed thereunder. Thus, it is difficult to control charge transfer efficiency in a charge path under the P-type semiconductor region, which is disposed to separate two transfer gate structures, independently of an impurity concentration of the charge holding portion. Therefore, if high priority is put on the performance of the charge holding portion, it is difficult to increase transfer efficiency in a charge path extending from the charge holding portion to a sense node.
The present invention has been made on the basis of this problem, and is directed to increasing charge transfer efficiency from a charge holding portion to an FD regardless of an impurity concentration of the charge holding portion.